High frequency active circuits, designed to operate at a frequency above 50 GHz, may be vulnerable to damage for a number of reasons. Such circuits may include features with extremely small physical dimensions, such as a few nanometers, that may be fragile, especially in areas around transistor gates. In addition, interconnects and passive components, such as capacitors and thin-film resistors, on a front-side of a circuit may be sensitive to scratches. Therefore, such circuits are at risk of damage when handled during packaging and assembly. Also, the front-side of the circuit may be exposed to humidity and other environmental conditions that may cause performance degradation or accelerate failure of active components.
Encapsulation, or passivation, has been used to provide some physical protection for active areas, interconnects and passive components on the front-sides of micro-electronic chips, using a layer or coating of a material having a higher dielectric constant than air. Examples of dielectric materials used for this purpose include BCB (benzocyclobutene), spin-on glass and polyimide. The protective dielectric material can also be used to provide mechanical support for multi-layer interconnects, allowing additional flexibility in the design of the circuit. However, the presence of dielectric material around gates of components of the circuit causes parasitic capacitance to increase. Parasitic capacitance can cause a significant degradation in the performance of the circuit, particularly at high frequencies, potentially affecting one or more of gain, frequency drift, noise, output power and power-added efficiency. For example, a GaN (gallium nitride) amplifier encapsulated with BCB can show a deterioration of approximately 1.5 dB in small signal gain and a shift in operational frequency of 10-15%, when compared with an unencapsulated amplifier that is otherwise identical. Such decreased performance can occur even where a low-loss dielectric material is used for encapsulation.
Therefore, the protection and flexibility provided by conventional methods is offset by a decrease in the performance of the circuit. In low frequency circuits, operating below 30 GHz, the benefits may outweigh the degradation in performance. However, at higher frequencies, above 30 GHz, the reduction in gain, output power and efficiency of the circuit can be problematical. Hence, the usefulness of conventional dielectric encapsulation and passivation protection methods for high frequency circuits is limited.